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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 13 August 2014 by John Abraham

We have long suspected that greenhouse gases which cause the Earth to warm would lead to a wetter atmosphere. The latest research published by Eul-Seok Chung, Brian Soden, and colleagues provides new insight into what was thought to be an old problem. In doing so, they experimentally verified what climate models have been predicting. The models got it right… again.

To be clear, this paper does not prove that water vapor is a greenhouse gas. We have known that for years. Nevertheless, the paper make a very nice contribution. The authors show that the long-term increase in water vapor in the upper troposphere cannot have resulted from natural causes – it is clearly human caused. This conclusion is stated in the abstract,

Our analysis demonstrates that the upper-tropospheric moistening observed over the period 1979–2005 cannot be explained by natural causes and results principally from an anthropogenic warming of the climate. By attributing the observed increase directly to human activities, this study verifies the presence of the largest known feedback mechanism for amplifying anthropogenic climate change.

As stated earlier, climate models have predicted this moistening – before observations were available. In fact, the models predicted that the upper troposphere would moisten more than the lower atmospheric layers. As the authors state,

Given the importance of upper-tropospheric water vapor, a direct verification of its feedback is critical to establishing the credibility of model projections of anthropogenic climate change.

To complete the experiments, the authors used satellite measurements of radiant heat. The emissions have changed but it wasn’t clear why they have changed. Changes could be caused by increases in temperature or from increased water vapor. To separate the potential effects, the authors compared the first set of experiments with others made at a different wavelength. That comparison provided a direct measure of the separate effect of moistening.

Next, the authors used the world’s best climate models to test whether the observed trends could be caused by natural changes in the Earth’s climate or whether they require a human influence. Sure enough, only the calculations that included human-emitted greenhouse gases matched the observations. The authors conclude that,

Concerning the satellite-derived moistening trend in recent decades, the relations of trend and associated range among three experiments lead to the conclusion that an increase in anthropogenic greenhouse gases is the main cause of increased moistening in the upper troposphere.

The authors then went further by showing that their computed results encompass third-party measurements only when the impacts of human-emitted greenhouse gases are included.

I chuckled when I asked Dr. Andrew Dessler about this study, and he told me,

The finding may be bigger than it looks, becasue a wetter atmosphere should create greater ice deposition on Antarctica. Is there evidence of this? Does it contribute to greater sea ice there? Is the increased rate of deposition greater than the increased rate of ice loss... therefore, is there a net gain or loss of sea level from Antarctica? Finally, what are the seismic implications? It seems they would be major. The planet has to retain its spheroid shape, it there is substantial disruption in one place there have to be adjustments, and probably internal heat contributions from the load differences. Consider that one millimeter of ice over 14m km is 14 cubic cilometers, or 14 billion tons.

I was just looking up this very figure the other day. According to everything I've read, that atmosphere can hold 7% moisture for every 1C increase in average temp. Since we have increased the average temp by ~0.8C from pre-industrial, your 6% figure is about right.

I thought the 7% figure was originally attibuted to Trenberth, but couldn't trace it to him specifically.

The link between Antarctic sea ice and land ice is nebulous as far as I know from the literature I have seen. The two things seem to be controlled by different things. See the relevant posts here.

That same post indicates that, despite the apparent increase in water vapor, Antarctica appears to have been a net conrtibutor to global sea level — i.e., the melting effects are outstripping the possible increases in precipitation.

There could be seismic implication of sudden retreat of land glaciers, just as we are still seeing isostatic rebound from the retreat of the last glaciation. Ice accumulation is pretty slow however, so we're not likely to see huge effects from accumulation of snow. 1mm of ice on top of miles of ice already there will be barely noticeable at depth.

Check! Steve: However, the estimates for Antarctic contributions are less than clear. It could be that more of the sea level rise is from Arctic changes, in which case Antarctica might be a neutral to date, or even increasing. Links to evidence would be most interesting now. My intuitive fear is that Antarctica can contribute more either way than we are estimating. I can see West Antarctica contributing water, and East Antarctica absorbing it in big numbers; we just don't know.

One mm is not insignificant. The spheroid will adjust, as per GIA, rebound, you point out. For perspective, 10 inches is equal to the mass of 3500 three gorges dams (TGDs). Ten inches would be historical for Antarctica, but we are in geologically historical times.

From my work on Pluvinergy, rhythmic GIA over millennia is a better candidate for tectonic plate movement motivation than liquid core circulation. Sorry, my work is all conjectural, but it is fun to think about, and frankly, it is a lot more convincing than conventional theory. PDF for the argument available if anyone is interested.

It is disconcerting that this informative discussion of a climate change issue is misleading in using the term 'anthropogenic'. It implies that humans have produced the emissions that are contibutng to climate change. People have only made (bad) intangible decisions. It is tangible technolgical systems that have done the damage. Better understanding of this causative factor could well lead to improvements in coping with the effect of climate change.

Sorry, but to me these arguments always read as sophistry at best, and in this case it doesn't even rise to that level. There are plenty of technological systems which don't contribute to global warming... and indeed, our only hope of stopping the process lies in developing cleaner technologies like wind and solar power. Humans are causing global warming. We actually started doing so thru land use changes before we even developed modern technology. 'Anthropogenic global warming' is an accurate term. 'Fossil fuel driven global warming' is an accurate term. 'Technological system driven global warming' is not, at least for some technological systems.

One interesting side effect of this upper tropospheric moistening is that since the HCH bending vibrations of methane are overwhelmed by the HOH bend of water, the warming effects of increasing methane concentrations should be somewhat mitigated by higher humidity. Does anyone know if model projections show this effect?

Ian F: I got a real nice response from Snow and Ice Center. They detailed that in 2010 the loss was 134 Gt, 3 in 2011, and 23 in 2012, and that it is now losing 159 Gt per year.

It seems we need to keep good funding to our satellite and scientific talent. It is nice to have these measuring capabilities, and it would be a shame to let them degrade when they are most needed.

It is reassuring that we have people who study and understand these issues well. Although, this is a little frightening that Antarctica does not seem to be helping us, as much as guessed that it could. It seems intutiively, that it should be a moisture sink, rather than a net contributor.

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